In general, a linear polymer having an amide bond as a repeating unit in the molecule is referred to as a polyamide. In particular, a polyamide having an aliphatic chain as a main chain is widely distributed as a resin with a trade name “nylon”. Typical examples thereof include nylon 66, nylon 6 and the like. A polyamide resin is a crystalline polymer, in which a hydrogen bond can be formed between amide bonds in different polymer chains. The crystal structure thereof is constituted in such a manner that the above-mentioned amide bonds are hydrogen bonded regularly in the same plane and simultaneously, each plane is arranged in a layered pattern, therefore, it is robust. Because of the polarity due to this amide bond and the above-mentioned crystal structure, a polyamide resin has an excellent resistance to a hydrocarbon-based solvent such as gasoline or oil. In addition, its thermal resistance is relatively high, and moreover, it has material strength, therefore, it is widely used as a preferred resin material for automobile parts, machine parts and the like. In order to form a part having a shape that cannot be formed by a single die molding process (e.g., an undercut structure), it is necessary that the parts with a corresponding shape be formed with a polyamide resin individually. However, in order to obtain strong joining strength, a method of treating the predetermined joining face with heat and joining polyamide resins together by close to welding has been employed conventionally.
However, for example, by using nylon 6, which is a polyamide resin, when nylon 6 in a melted state, which becomes an additional molded portion, is injected and filled to an existing molded article, which is made of nylon 6 and is placed in a metal mold, to try to join them together in accordance with the above-mentioned method, both are joined together, however, the joining strength is not sufficient. In addition, in the case where they are cooled down without being joined together, they will be separated in some cases. The cause is as follows. Some of the amide bonds in the polymer chain of nylon 6 forming a predetermined joining face that is an end surface of the existing molded article are in a free state in which they do not form a hydrogen bond because there is no polymer chain, which is an adjacent partner to be mutually hydrogen bonded in the same plane as described above.
In addition, in order to inject and fill a melted resin of nylon 6, which becomes an additional molded portion, in a metal mold, it is necessary to carry out a preliminary preparation by heating this metal mold and maintaining it at a predetermined temperature. However, the above-mentioned amide bonds in a free state in the nylon 6 forming the predetermined joining face of the above-mentioned existing molded article are trying to find a partner for a hydrogen bond for association as best they can during this heat treatment, and randomly form a hydrogen bond in an amorphous state within the polymer chains in the nylon 6 of their own.
In this way, the amide bonds in the predetermined joining face of the existing molded article to be hydrogen bonded essentially with the amide bonds in the nylon 6 in a melted state in the additional molded portion are placed in a metal mold in a state in which they already form a hydrogen bond within the polymer chains in the nylon 6 of their own, though they are in an amorphous state. Therefore, the amide bonds in the nylon 6 in a melted state in the additional molded portion to be injected in the metal mold are mutually hydrogen bonded within the polymer chains of their own as they are cooled in the metal mold after the injection. As a result, the nylon 6 in the existing molded article and the nylon 6 in the melted resin are crystallized individually, therefore, it is difficult to join them.
The same problem applies to the case where polyamide resin molded articles are subjected to ultrasonic welding. More specifically, the vibration energy caused by ultrasonic waves is directed to both of the predetermined joining faces, and the polyamide resins in the area are slightly melted by heating and mutually hydrogen bonded. Then, being cooled down, they are supposed to be joined together, however, in the process of the heat treatment, each polyamide resin molded article is crystallized individually as described above. Accordingly, in order to sufficiently weld them by directing the energy more efficiently, the above-mentioned predetermined joining face is subjected to a treatment such as a precise processing of a number of edges for ensuring the area to be contacted. Both polyamide resin molded articles are joined together, however, it is not sufficient in terms of the joining strength.
Incidentally, as a joining method that provides an appropriate joining strength by using a polyamide resin molded article, there is a resistance welding method. It is a method for molding of incorporating a conductive electric wire into an article to be molded along the outline of the article, turning on electricity thereby allowing the conductive electric wire to generate heat and to become a hot wire, and while melting the article to be molded, injecting and filling a melted polyamide resin on the melted article to be molded in the metal mold. However, the efficiency of the operation of incorporating a conductive electric wire is very low, which results in a high production cost.
Accordingly, an object of the present invention is to join polyamide resins together with sufficient joining strength.